There are many patents on expendable cores for casting dies with various compositions of coatings for the dies and the cores therefor to withstand the hot metals poured or injected into the dies until the metals are solidified. Even applicant's assignee owns the above mentioned related patents and patent application which disclose expendable sand cores having coatings, which cores have good shake-out properties, good wash-out resistance, good resistance to surface penetration, good shelf-life, and high core strength to withstand pressures in excess of 1000 psi to form smooth undercut regions in die castings.
Nevertheless, still higher pressures in die casting processes are often desired and therefore it is essential to increase the strength of the expendable cores used therein. Applicant, after considerable experimenting and testing, has unexpectedly discovered new compositions and additives for the coatings of such expendable sand cores for die casting dies, which coated cores have improved physical properties over what has been known before; namely to withstand greater pressures and hotter metal melts, and to produce smoother undercut portions in die castings.